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Sophisticated characterization capabilities are essential in order to satisfactorily analyze high purity semiconductor materials. The electrical properties of semiconductors have a long history of extensive investigation. As the investigations of semiconductors were extended to wider band gap materials electrical measurements were not as readily applicable. This coupled with the understanding of excitons and their contribution to the elucidation of materials properties in the 1960's lead to a wide application of optical studies to semiconductor materials. It was found that these materials reflect, absorb, disperse, scatter and radiate light and in general interact strongly with the electromagnetic radiation field. Because of this strong interaction many of the fundamental properties of these materials such as their energy band gaps, activation energies of defects and foreign impurities, effective mass parameters, refractive indicies, dielectric functions, exciton binding energies and lattice vibration frequencies can be determined from optical experiments. The technique of high resolution optical absorption, reflection and photoluminescence spectroscopy has been extensively used to analyze the intrinsic energy band parameters of semiconductors, as well as their impurity and defect states. Intrinsic or free exciton formation is observed in most well formed crystal structures when optically excited with the proper energy and at cryogenic temperatures. The free excitons have been applied with a great deal of success in probing the intrinsic band structure of semiconductors. Bound excitons have been applied successfully in probing the impurity and defect structure of many of these same materials. The free exciton is the probe in this case, becoming bound to various chemical impurities, lattice defects and complexes to form bound states whose sub-sequent radiative decay yields information concerning the electronic states of the impurities and defects in these materials. Magnetic field splittings of the bound exciton transitions make it possible to differentiate between neutral and ionized donor and acceptor impurities. In conjunction with systematic impurity doping experiments, specific donor and acceptor impurities can be identified. Upon application of stress fields, in conjunction with a knowledge of the energy band structure of the host lattice, it is also possible to differentiate between simple substitutional donors and acceptors and complexes comprising combinations of impurities and/or defects.
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